Earth-moving or crawler-type tractors are conventionally equipped with endless tracks on opposite sides of the tractor. It is customary that each endless track be trained over a driving sprocket, usually disposed to the rear of the tractor, and a forwardly disposed idler sprocket. The driving and idler sprockets are mounted on a track frame.
The idler sprocket is normally mounted to the track frame for longitudinal sliding movement relative thereto. The longitudinal sliding movement of the idler sprocket serves to maintain the track under proper tension and also allows for some measure of recoil should rocks, stones, or other foreign materials become lodged on the inside of the track. Idler sprocket mounts have typically been provided with slides and guides to facilitate the longitudinal sliding movement between the idler sprocket and the track frame.
Interfacing sliding surfaces on the idler sprocket mounts and track frame are subject to substantial wear because of relatively high shocks and stresses transmitted and imposed thereon during operation of the tractor. Moreover, grit, dirt, mud, and other abrasive foreign matter entering between the interfacing sliding surfaces on the idler sprocket mount and track frame during operation of the tractor accelerate the wear between the parts.
The amount or degree of wear in the area of sliding contact between the idler sprocket mount and track frame is proportional to the vertical movement or bounce of the idler sprocket relative to the frame. Of course, vibration or bounce of the idler sprocket exacerbates the wear problem between the parts. Vertical vibrations or bounce of the idler sprocket furthermore generates noise and causes operational control problems including coarseness of the ride and steering problems which hinder or limit tractor ground speed.
It is desirable, therefore, to eliminate vertical movement of the idler sprocket relative to the frame. One method of accomplishing this end is to replace the worn parts. As will be appreciated, however, replacement of the parts is costly besides the considerable time that must be expended to disassemble and reassemble the idler sprocket mounts. Moreover, changing the worn parts is difficult to accomplish in the field and requires an inventory of replacement parts.
Alternatively, shims are used to compensate for the increased vertical clearance resulting from wear between interfacing sliding surfaces on the parts. Shims are conventionally secured in place by bolts received in slots provided on the idler sprocket mounts. Ice, mud, rocks, and other material which accumulate on the tractor frame add to the frustration of mounting the shims in place to compensate for wear between the parts. Moreover, the use of shims is subject to error because of the human factor involved.
Using shims to eliminate vertical vibrations of the idler sprocket is also expensive due to the drilling and tapping procedure for the shims. Some consider this approach uneconomical in that the shims must be thick enough such that the threaded holes for the bolts, used to secure the shims in place, must be vertically removed in relation to the wear surface of the shim. As will be appreciated, a bolt passing through the wear surface of the shim will accelerate the wear between the interfacing sliding surfaces. Thus, additional time needs to be expended during maintenance to assure that the shims, spacers, or the like do not require replacement themselves.
U.S. Pat. No. 3,907,382 assigned to Caterpillar Tractor Co. represents still another alternative for inhibiting vertical bounce of the idler sprocket relative to the track frame. As illustrated, a series of fluid actuated pistons affect the disposition of interfacing wear surfaces on the idler sprocket mount and the track frame relative to each other. Each piston is arranged for reciprocal endwise movement within a normally closed fluid chamber. Fluid is introduced into the chamber and behind the piston through a fitting.
An inherent characteristic of fluids is that they are incompressible. Moreover, during tractor operation, the rough terrain over which the tractor moves inherently imparts relatively high impact forces between the idler sprocket mount and the track frame. Such relatively high impact forces are likewise transmitted and tend to vertically move the pistons into the closed fluid chamber.
Upward movement of the pistons against the incompressible fluid prompts fluid leaks in the system. As will be appreciated, fluid leaks are an inherent problem with any fluid system. Moreover, and because the fluid is normally entrapped within a closed chamber, only limited pressure can be applied to the pistons by the incompressible fluid. Wear between the parts and fluid leaks resulting in loss of fluid from the closed chamber significantly reduces the fluid pressure applied to the pistons thereby adversely effecting the ability of the pistons to inhibit vertical bounce of the idler sprocket. Fluid leakage problems along with limited pressure characteristics furthermore requires regular maintenance for such a system.
Thus, there is a need and a desire for an idler sprocket mount that allows for sliding longitudinal movement of the idler sprocket and automatically compensates for wear between interfacing sliding surfaces thereby reducing noise and vibrations and overall handling characteristics of the tractor.